Mechanical couplings for joining pipe elements together end-to-end are often preferred over more permanent methods for joining pipe, such as welding or brazing. Mechanical couplings allow for the rapid assembly and modification of piping networks by mechanical technicians without specialized welding skills or the need for fire watches or hot work permits. The term “pipe element” is used herein to describe any pipe-like item or component having a pipe-like form. Pipe elements include pipe stock, pipe fittings such as elbows, caps and tees as well as fluid control components such as valves, reducers, strainers, restrictors, pressure regulators and the like.
Prior art mechanical couplings for joining pipe elements together end-to-end may comprise interconnectable segments that are positionable circumferentially surrounding the end portions of co-axially aligned pipe elements. Each mechanical coupling segment comprises a bridge structure having arcuate surfaces which project radially inwardly and engage plain end pipe elements or circumferential grooves that extend around each of the pipe elements to be joined. Engagement between the arcuate surfaces and the pipe elements provides mechanical restraint to the joint and ensures that the pipe elements remain coupled even under high internal pressure and external forces. The bridge structure defines an annular channel that receives a gasket or seal, typically an elastomeric ring which engages the ends of each pipe element and cooperates with the segments to provide a fluid tight seal.
While prior art couplings are effective, they exhibit certain drawbacks. For example, when such couplings are used with fittings such as elbows and tees, a separate coupling is needed at each connection point, rendering such points heavy and bulky. The couplings also cause a significant increase in the center to end dimension of the fitting. Furthermore, the seals used with these couplings tend to be large, and if made from expensive materials such as fluorinated elastomers, the coupling assembly may be prohibitively expensive.
Prior art mechanical fittings that are specifically designed to avoid these disadvantages, such as threaded fittings, bring other problems. For example, it is well known that threaded coupling systems suffer a relatively high initial leak rate, wherein as many as 8% of the joints may leak when first tested under pressure. Threaded systems also tend to be relatively expensive due to the need to cut threads in the pipes and fittings to be joined. Additionally, for larger diameter pipes, threaded systems require high torque to effect a good connection. Threaded systems also require special fittings known as unions to be used throughout a network in order to allow specific portions of the network to be disassembled without disassembling the entire system. This is due to the threaded nature of such systems where turning a pipe to disengage it from one fitting tightens the fitting at the opposite end and prevents removal of the pipe. Pipes in a threaded system need to be of sufficient thickness to cut threads into the pipe sidewall. Thus, thin-walled pipe elements cannot be used when the design requires, resulting in an overweight, heavy piping network with pipes having sidewalls far thicker than necessary to bear the internal pressure or external loads expected. Furthermore, there are always exposed threads adjacent to every fitting engaged by a pipe. These exposed threads comprise a weak point for bending strength and allow for increased failure due to corrosion because they constitute the thinnest region of the pipe sidewall and provide a source of stress concentration. There are clear advantages to be realized by using mechanical pipe couplings which do not suffer such drawbacks.